The present invention relates to gas turbine engines and, more particularly, to high temperature turbine shrouds.
Heat generated in gas turbine engines presents a challenge to gas turbine engine structural designers. This problem is particularly challenging in the turbine portion of the engine where temperatures generated by the upstream combustor are most severe. For example, turbine flow path defining members are subjected to the products of combustion, and as combustor temperatures increase to levels well in excess of 2000.degree. F, it becomes increasingly difficult to provide a structure which can withstand such an environment. The problem is further compounded in gas turbine engines for aircraft application where light weight is an essential design consideration.
Basically, two approaches have been taken to at least partially alleviate the problem. On the one hand, various methods of fluid cooling of the turbine structure have been employed. Such methods are commonly referred to as convection, impingement and film cooling. These methods are well known in the art as discussed in U.S. Pat. No. 3,800,864 -- Hauser et al, which is assigned to the assignee of the present invention. All of these methods tend to reduce peak metal temperatures and temperature gradients through the use of a cooling fluid (typically air), thereby making the use of higher turbine inlet temperatures possible. However, as the combustion temperatures increase, the amount of cooling air necessary to maintain acceptable metal temperatures also increases. Since cooling air is air which could otherwise be effectively used in the propulsion cycle, and since the extraction of the air for cooling purposes effectively penalizes the overall engine performance and efficiency, it is desirable to keep the coolant flow at as low a level as possible.
The second approach toward obtaining a high turbine operating temperature is to provide a material which can withstand the high temperatures with little or no fluid cooling. One type of material which can endure such temperatures is ceramics. Modern, hot pressed ceramics such as silicone carbide and silicone nitride possess very high strength at elevated temperatures. It is now contemplated that this high temperature strength can be utilized to design gas turbine components requiring little or no cooling air to properly function at allowable levels of thermal stress, thereby enhancing overall turbine efficiency and performance. The low costs and light weight associated with ceramics offer additional advantages in aircraft gas turbine engine applications.
Certain characteristics of ceramic materials must be addressed, however, prior to executing a successful design in ceramics. In particular, it must be recognized that ceramics are brittle materials, having little or no ductility and low impact tolerance. Furthermore, the thermal expansion coefficients of ceramic materials are only about 10 to 20 percent of conventional nickel alloy materials, thereby presenting interface problems compounded by the ceramic material's relatively low level of tensile strength.